Crude oil prices in 2018 saw their first annual decline since 2015, the U.S.' Energy Information Administration (EIA) said in a statement on Thursday.

International benchmark Brent crude ended 2018 at around $54 per barrel, or approximately $13 a barrel lower than it began the year, the EIA said.

American benchmark West Texas Intermediate (WTI) finished last year at $45 a barrel -- $15 per barrel less than it started 2018, it added.

"This year [2018] marks the first time since 2015 that crude oil prices for these benchmarks ended the year at a lower price than at the beginning of the year," the statement said.

During 2018, both benchmarks climbed to their highest levels on Oct. 3 when they hit approximately $86 per barrel and $76 per barrel, respectively.

However, crude oil prices plummeted quickly after that. On Dec. 24, Brent crude fell to its annual low of $50 per barrel and WTI declined to its lowest on an annual basis at $43 a barrel.

- US crude hitting record

One of the main reasons behind the sharp decline in oil prices last year was the rise in crude oil production in the U.S.

The U.S. surpassed Russia and Saudi Arabia in 2018 to become the world's top crude producer with 11.7 million barrels per day (mbpd) of output for the week ending Nov. 9, according to the EIA data.

In addition, "U.S. crude oil exports averaged 1.9 mbpd in 2018, about twice the amount that was exported in 2017," the statement said.

"The growth in U.S. exports of crude oil and petroleum products led to a one-week period in late November when the U.S. was a net exporter for the first time in EIA’s data history," it added.

- OPEC output down slightly

The EIA said it estimates that OPEC's total crude oil and other liquids production will average 39.2 mbpd in 2018, down slightly from 39.3 mbpd in 2017.

Crude oil production increased in Saudi Arabia during the second half of 2018, which partially offset Iran’s declining production as a result of U.S. sanctions.

Although U.S. sanctions on Iran began to be re-implemented on Nov. 5, Washington granted waivers to eight countries to continue buying oil from Iran for six months -- a move that increased supply in the global oil market and pushed crude prices lower.

Russian crude oil and other liquids production increased from an estimated 11.2 mbpd in January to an estimated 11.6 mbpd in December, the EIA said.

Saudi-led OPEC and Russia-led non-OPEC agreed on Dec. 7 to curb their total production by 1.2 mbpd starting from Jan .1 for the first six months of 2019 in order to help trim some of the oversupply in the market.